what is the highest wattage bulb the lamp kit can handle?

The cord and socket is not usually the limiting factor on maximum wattage. I don't know what guidelines come with the kit, but they should be followed. Sockets like this are typically rated around 500 watts, however, the materials and design of the shade usually limit the wattage to a much lower level. I would start with 60 watts and if you need more light, move up to larger bulbs. Keep checking the temperature of your shade, and if it feels or smells hot, then back off to a smaller bulb.